For treatment of various problems with the shoulder and hip such as degenerative arthritis and trauma, one method of providing relief to a patient is to replace the articulating surfaces of a diseased joint with an artificial or prosthetic joint. In the case of a shoulder, the humerus and glenoid articulating surfaces may be replaced. In the case of a hip, the femur and acetabulum articulating surfaces may be replaced.
In such replacement, pain relief, increased motion and anatomic reconstruction of the shoulder or hip joint are goals of the orthopaedic surgeon. With multiple variations in human anatomy, prosthetic systems need to accurately replicate the joints that they replace and maintain the natural symmetry of the joints on the left and right sides of the patient's body.
A hip replacement procedure may involve a total hip replacement or a partial hip replacement. In a total hip replacement procedure, a femoral component having a head portion is utilized to replace the natural head portion of the thighbone or femur. The femoral component typically has an elongated intramedullary stem which is utilized to secure the femoral component to the patient's femur. In such a total hip replacement procedure, the natural bearing surface of the acetabulum is resurfaced or otherwise replaced with a cup-shaped acetabular component that provides a bearing surface for the head portion of the femoral component.
Acetabular cups may be secured to the acetabulum in a number of different ways. For example, an acetabular cup may be secured to the acetabulum by the use of bone cement. However, recent studies have speculated that it may be desirable to secure artificial components to natural bone structures without the use of bone cement. Hence, a number of press fit acetabular cups have been designed for implantation without the use of cement.
Regardless as to whether cement is used or not, a surgeon typically uses a surgical tool called a reamer to ream the acetabulum and create a cavity into which the acetabular cup is secured. One problem that surgeons encounter is being able to properly match the size of the reamer to the desired acetabular cup size.
Although press fitted acetabular cups are typically described as being “generally hemispherical” in shape, such cups, in reality, are sub-hemispherical. A sub-hemispherical configuration suffers from a number of limitations. For example, a surgeon may have difficulty in reaming a properly sized cavity for an acetabular cup that is not truly hemispherical. This difficulty arises because the cutting heads of reamers are typically configured as relatively true hemispheres. Hence, when a surgeon reams the patient's acetabulum, the surgeon has to “estimate” the approximate depth of the reamed recess. If the surgeon reams too much of the bone, the annular rim of the acetabular cup sits too deeply within the cavity. Conversely, if the surgeon reams too shallowly (i.e. “under reams”), the acetabular cup does not fully seat in the reamed cavity of the acetabulum. Because a surgeon may occasionally select a reamer that is slightly smaller than the acetabular cup to be implanted, under reaming may also disadvantageously lead to bone fracture of the acetabulum since excessive force may be used to insert the cup into the undersized (i.e., under reamed) cavity. Some of the early bone cups held in place by cement did not suffer from this problem because they were more accurately shaped as “true” hemispheres. However, the use of cups requiring cement for implantation is now considered undesirable.
Another drawback associated with press fitted acetabular cups relates to the configuration of the outer shell. In particular, in an attempt to increase retaining forces, a number of acetabular cups have been designed with a flared rim (known as dual radius or “bubble” cups) or a frusto-conically shaped annular rim portion (known as dual-geometry cups). Although the configuration of such cups may generate relatively strong retention forces at the rim portion of the cup, surface contact and therefore retention forces are relatively small at the portions of the outer shell other than the rim portion, particularly in the dome area. Moreover, such reduced surface contact at the portions of the outer shell other than the rim portion reduces bone ingrowth in such portions.
With the above-mentioned press fitted acetabular cups, a two-part reaming process is typically necessary. The two-part reaming process involves reaming the acetabulum with a relatively small reamer and then increasing the size of the cavity with a larger reamer. The more reaming, the more likely that a problem will occur. For example, many conventional cementless acetabular cup systems use a cup that is two millimeters larger than the last reamer size used. Inserting this size cup into the undersized reamed acetabulum to accommodate this system is sometimes difficult, particularly with resistance in the dome area of the cup, which is also larger than the last reamer size used.
Generic forms of acetabular cups are more difficult to accurately place in a joint because they are not uniquely designed and manufactured to conform to a patient's joint structure. One factor affecting accurate acetabular cup placement is the frequency with which some orthopedic surgeons encounter joint replacement surgery. Demographics in some regions may make joint replacement surgery in some regions a more common component of a surgeon's practice. However, a majority of the joint replacement surgeries are performed by surgeons for whom such surgeries are an occasional rather than stable component of their practice. That is, their relative lack of experience makes proper component placement more difficult for them as they are not as familiar with the anatomical landmarks and how they are to be aligned with the prosthetic components. Proper sighting is encumbered by the alteration of the joint's appearance by reaming and other surgical procedures required to prepare the joint for implantation of the replacement parts.
Proper orientation and implantation of generic prosthetic components may be facilitated by use of interoperative guides or templates. A guide may be a jig that is designed to temporarily attach to anatomical landmarks in the region of the joint and is configured to permit a prosthetic component to be inserted through an opening in only one orientation. Thus, the jig assists a surgeon in correlating the view of the surgical sight through the incision to the pre-operative template. A pre-operative template is an image or model of the patient's joint that is staged to take the surgeon from the state of the joint upon entry through the incision to its preparatory stages to the final scene of the implanted prosthesis.
In order for a jig to be an effective aid, however, it must first be accurately placed in the area being reconstructed. The attachment structure of the jig must be configured so it mates to unique enough bone structure to ensure its proper placement with respect to the surgical site without interfering with the working area at the site. If the jig is made with an assumption that patient anatomical structure is generally the same, then the jig may not be placed accurately enough to ensure that component orientation is optimal within the joint. Templates, while useful for pre-surgery planning and development of a surgical plan, may not be useful once the surgeon begins reconstructive work on the joint. Alteration of the surgical site by reaming and positioning of surgical instruments within the site may obscure the surgeon's view. Thus, the surgeon may have trouble envisioning how the reconstructed joint is conforming to the pre-operative plan embodied in a template.
Consequently, there is a need for a method of designing and manufacturing a surgical guide that addresses these and other drawbacks. With respect to the hip joint, there is a particular need for a surgical guide, such as a jig, and associated method of making a surgical guide that overcomes one or more of the above-mentioned problems. More particularly, what is needed is a surgical guide for placement of an acetabular prosthesis and associated method of fabrication that enables the acetabular cup to be properly oriented with respect to the reconstructed acetabulum of the innominate bone.
There is also a need for a surgical guide and associated method of fabrication that provides structure for accurately placing the jig in the vicinity of the surgical site for proper orientation of a replacement component without interfering with a surgeon's access or view of the site. Again, with respect to the hip, a need remains for a surgical guide for an acetabular prosthesis and associated method of fabrication that provides. proper angular orientation of an acetabular cup within a reconstructed acetabulum of the innominate bone without impeding access to the reconstruction area or its view.